Memory CD8 T cells can provide exceptionally long-term protective immunity against reinfection due to their enhanced longevity, proliferative capacity and functional potential. During a viral infection, the activated CD8 T cells clonally expand and acquire cytotoxic function to eliminate infection. However, after viral clearance, antiviral effector CD8 T cells rest down and stop dividing and synthesizing antiviral proteins. A portion of the effector cells differentiate and mature into protective memory CD8 T cells that are long-lived, have a high proliferative potential and can replenish the memory pool by homeostatic proliferation. Recent work suggests that this effector`memory (E`M) transition requires a metabolic switch from an anabolic, rapidly dividing state that relies primarily on glucose and glycolysis (driven by the PI3K/AKT/mTOR pathway) to a catabolic, quiescent state that relies on fatty acids/ fatty acid oxidation (FAO; driven by Foxo/AMPK activity). However, this model requires substantial investigation because very little is known about memory CD8 T cell metabolic states, which signals regulate this process and how this affects their lifespan and function. Perhaps more importantly, we need to better understand how changes in CD8 T cell metabolism are coordinated with T cell differentiation during immune responses. We hypothesize that STAT3 lies at the cross roads of these two processes because it is both a transcription factor that controls memory T cell differentiation and a mitochondrial protein that modulates mitochondrial respiration. Our preliminary data show that IL-21/IL-10/STAT3 signaling is essential for memory CD8+ T cell development during an acute viral infection. In absence of either IL-21 and IL-10 or STAT3, virus-specific CD8+ T cells retain terminal effector (TE) differentiation states and fail to mature into functional memory T cells that protect against reinfection and contain self-renewing TCM cells. Thus, STAT3 activity drives the E`M transition and is necessary for functional, protective memory CD8 T cells to form after acute LCMV infection. Additionally, we have data suggesting that STAT3 controls both the expression of key transcription factors involved in memory CD8 T cell differentiation and the activity of the PI3K/AKT/mTOR and AMPK pathways that regulate memory T cell metabolism, differentiation, function and survival. These data suggest STAT3 integrates various signals and coordinately regulates both T cell differentiation and metabolism. Our primary aim in this proposal is to determine if STAT3 regulates an anabolic ` catabolic switch during memory CD8 T cell differentiation. To study this, we will determine whether (1) PI3K/AKT/mTOR activity is abnormally increased and, reciprocally, if AMPK activity is decreased in Stat3-/- virus-specific CD8 T cells and (2) if STAT3 controls glycolysis, oxidative respiration and fatty acid oxidation in virus-specific CD8 T cells. Lastly, we will (3) compare the roles of nuclear and mitochondrial STAT3 in memory CD8 T cell development, metabolism and gene expression. ) ) PUBLIC HEALTH RELEVANCE: A T cell's metabolic state controls its ability to differentiate, proliferate and survive, and recent elegant studies have suggested that the ability of a memory CD8 T cell to form and persist depends on its ability to switch from an anabolic to catabolic state, utilizing fatty acid oxidation as the major mode of ATP production. This grant will examine the dual roles that STAT3 plays in this process both as a transcription factor and a mitochondrial protein to coordinate the differentiation and metabolism of memory CD8 T cells. This work could uncover multiple novel mechanisms for regulating lymphocyte metabolism and the generation of memory T cells that could enhance vaccine development and T cell based immunotherapies as well as our understanding of diabetes, obesity and aging. )